It seems there are some common defects for some lenses and cameras!

For some older photographers it is well known that some few lenses and cameras have typical defects or problems.
This could be the soft front lens on some lenses, or a camera capacitor that breaks after decades. But newer lenses have sometimes frequent problems as well.
For example I have read about some Canon EF super tele lenses with fungus (1, 2, 3 ) . This could probably be a result of a special coating, or more likely these lenses are more often used under humid enviroment than the cheaper lenses most amateurs use.

Here some first ideas about common problems, with some references.
This is not a scientific proof of bad work or at the manufacturer - some things only appear after decades!

At the moment I want not to list the very common defects like Canon EOS 5D mirror, Sigma iris problem with old lenses and new cameras and such. Some of the references may not show the pictures to non-forum-members, sorry for this.